Weeks 7-8 Notes

Week 7 - Understanding the Resting Membrane Potential

Law of Macroscopic Electroneutralilty: overall, an ion solution is electrically neutral

• extracellular fluid and the cytosol have the same number of + and - charges

• even though the thin layers differ, the overall charge is neutral

  • thin layer on the edge of the cytosol is negative

  • thin layer on the extracellular membrane is positive

• ex. if the entire class is a 50:50 ratio of males to females, it does not necessarily mean each row has a 50:50 ratio of males to females

Volt is a unit of electrical potential difference

Voltmeter is used to measure the difference between voltages

• by convention, resting membrane potential is measured as “inside” compared to “outside” to minimize confusion

• if the voltmeter reads -70mV, that means the recording electrode (on the thin layer of the cytosol) is -70mV compared to the reference electrode (on the thin layer of the extracellular fluid)

• intracellular recording

Extracellular recording is when the reference electrode and recording microelectrode are both on the extracellular layer, but they see changes in charge when the extracellular environment changes

• voltage difference will occur when ions are moving in and out

• measurement over time

All cells are polarized

• have a resting membrane potential

• not only neurons

• but the value differs among different cell types

• neurons have a resting membrane potential to communicate information

Free energy is inversely proportional to the electrical potential difference

• free energy is the energy available for work and biosynthesis

  • ex. the energy used to move an ion from one place to another, to circulate blood, etc.

  • resting membrane potential is an example of electrical potential difference

  • the closer the electrical potential difference is to 0 (small magnitude), the greater the release of free energy

  • free energy must be used (so starts to decrease) as the resting membrane potential increases

    • because it takes energy to pump ions against the concentration gradient to reach equilibrium

Sodium-Glucose Transporter:

• protein that brings glucose into a cell

  • even when the concentration of glucose is higher inside the cell than outside

  • against the concentration gradient

• when Na+ comes in, the inside becomes more positive, which decreases the magnitude of the membrane potential (originally it is more negatives, so adding positive ions makes it more neutralized)

• according to the free energy equation, if the membrane potential decreases, then more free energy is released

• this free energy is used to power glucose to come in, against the concentration gradient

• therefore, not all cellular work and biosynthesis is directly linked to ATP

  • in this case, the source is membrane potential

Resting Membrane Potential is established because the ion concentrations inside and outside differ

• extracellular is dominated by Na+ and Cl-

• cytosol is dominated by K+, PO43- and Pr (proteins)

  • proteins usually carry a -ve charge

Equilibirum Potential

• the value of membrane potential at which the net flux of ion across the membrane is zero

• two gradients (electrical and concentration) are balanced

• measured using the Nernst Equation, dependent on concentration, charge, and concentrations

  • as temperature increases, the magnitude of membrane potential also increases

  • increased temperatures cause molecules to move easily

  • if they move more, a bigger electrical gradient is required to oppose it

K+:

• resting membrane potential arises because of the ion gradient

  • concentration gradient will move the ions from inside to outside

  • eventually, the inside will become more negative, which will start to move the ions back inside with the electrical gradient

• higher concentration inside than outside

• if the equilibrium potential value is -89mV, then it means that K+ will leave until the inside becomes -89mV, which is when the electrical gradient will start pulling it back in at the same rate

• but this movement is very small compared the the large amount of ions in a cell, so the difference is negligible

  • not violating the Law, as the law is simply an approximation

If the membrane potential is not equal to the equilibrium potential, then net movement of ion across the membrane will occur

• if the membrane potential is -100mV, and the equilibrium potential is -89mV, then K+ must be pushed back in until it reaches -89mV to be at equilibrium

Na+:

• resting membrane potential arises because of the ion gradient

  • concentration gradient will move the ions from outside to inside

  • eventually, the inside will become more positive, which will start to move the ions back outside with the electrical gradient

• higher concentration outside than inside

• ex. +72mV

  • the value that the membrane potential is trying to reach for sodium

Cl-:

• resting membrane potential arises because of the ion gradient

  • concentration gradient will move the ions from outside to inside

  • eventually, the inside will become more negative, which will start to move the ions back outside with the electrical gradient

• ex. -84mV

  • the value that the membrane potential is trying to reach so that the net flux of Cl- is 0

Normally, cell membranes are impermeable to charged species

• hydrophobic core

• so these ions move through protein leak channels

  • always open

• K+ leak channels are more abundant because the relative ion permeability of K+ is 100, when Cl- is 10, and Na+ is 1

Roommate Analogy:

• when a man and a woman are roommates, woman prefer 85 degrees Fahrenheit where men prefer 65

  • just like how Na+ and K+ prefer different equilibrium potentials

• if rent is 50/50, it is fair that the temperature is set to 75, an even split in the middle

• but if the man pays more rent, the temperature should be set to a value close to the man’s preference

• but if the woman pays more, the temperature should be set to a value close to her preference

• it depends on who contributes more to the cost

At rest, neither Na+, K+ or Cl- are at equilibrium

• resting membrane potential is a steady state

Goldman Equation:

• determines the resting membrane potential based on all ions depending on how much they contribute (permeability)

• if the value is -85mV, that is closer to the equilibrium value of K+ because it has the highest permeability

Week 7 - Lab

Intracellular action potentials:

• intracellular action potential recordings allow us to examine the behaviour of a single excitable cell (like neurons or muscle fibers)

• but these are difficult to do

Extracellular action potentials:

• from single cells are possible but only when the cell is large and isolated from other cells (like earthworm nerve cord)

In most multicellular animals, multiple neurons are found in close association

• most nerves in our bodies are composed of dozens or hundreds of axons each

Compound action potential

• like a single action potential, there is a minimum threshold stimulation intensity below which no CAP will be elicited

• unlike a single action potential the amplitude of the CAP can change as stimulation intentisty is increased

  • CAP is made up of the roughly synchronous firing of multiple axons within the nerve

Conduction velocity:

• may differ per individual action potentials because some axons might be larger in diameter than others

• the average conduction velocity of the population of axons can be measured

Refractory period:

• following each firing, the neuronal membrane becomes briefly refractory

• absolute refractory period → the axon will not fire again even if it is stimulated with a suprathreshold stimulus

• relative refractory period → an axon will fire again, but only if it is stimulated with a suprathreshold stimulus and the amplitude of the action potential will be reduced

By delivering multiple stimulation pulses in succession, increasing there stimulation frequency (thus decreasing the delay between stimulation pauses), you will eventually see that a CAP no longer exists for each stimulation pulse.

• In summary, increasing the frequency of stimulation pulses reduces the time available for recovery between each pulse.

• When this happens, the nerve or muscle fibers can no longer generate individual action potentials, leading to the disappearance or reduction of the CAP.

The rising phase is when a large number of voltage-gated Na+ channels open and allow a net influx of sodium with the concentration gradient

• this causes the membrane potential to rise as it becomes more positive

The resting potential begins to fall (repolarization) when:

• voltage-gated K+ channels also are triggered at the same time but are slower to respond

  • offsets the influx of positive current from a new stimulus → leads to a relative refractory period as it contributes to a relative inability of this portion of the membrane to initiate a new action potential

• or voltage-gated Na+ channels stop permitting the flow of Na+ as their inactivation gates swing to the closed position

  • remain in the closed position until the membrane repolarization back below the threshold occurs, and the channels cannot reopen again

  • this leads to an absolute action potential

Both CAP conduction velocity and duration of the absolute refractory period can be affected by temperature.

Week 8 - Comparative Digestive Physiology

Variation in Food Quality:

• quality refers to how easily the food can be digested

• animal foods are typically high quality

• while plant foods are typically low quality except nectar

• refractory refers to resistance to degradation (digestion)

Animal foods contain a lower portion of compounds that are difficult to digest than plant foods

The type of diet is based on quantity vs. quality

• plant vegetation is probably the most abundant food source on Earth

• cellulose is the most abundant organic compound on Earth

• animal diets are high quality but less abundant while plant diets are low quality and more abundant

Type of Diet:

• most animals are omnivores

• most plants are also omnivores

• but the number of pure herbivores is greater than the number of pure herbivores

Evolution:

• it is difficult to change from one end of the spectrum to the other end

  • ex. herbivores to carnivores or carnivores to herbivores

  • so this is less common

• what is most common is the transition of herbivores becoming omnivores

• and next most common is carnivores to omnivores

• transition to omnivory has become evolutionarily favourable (frequent)

• as competition for specialized food resource increases, there is increased pressure to incorporate additional food resources

  • as the competition for fish in piscivores increases it’ll be more difficult to meet energetic demands so they start eating like an omnivore

  • population increase or prey number decrease can cause these shifts

Foraging Costs:

• generally, the bigger you are, the larger the home range you have to patrol

• herbivores: search costs are low and handling costs are low

  • but even considering body mass, herbivores tend to patrol less range than carnivores because plants are abundant so it is easy to find food

• carnivores: search costs are high and handling costs are high

  • it is harder to hunt animals

  • ex. chasing

  • chasing something smaller is easier but the reward is not enough, so the amount of small stuff you must chase is more compared to one big thing to get the same reward

• some plants are not as easy to digest

  • ex. thorns

  • some plants produce toxins too, so the handling cost is high because the predator must be able to detoxify it

Detoxification Limitation Hypothesis

• feeding is limited by the animals capacity to detoxify

• bitter taste → toxic

  • plants have a greater capacity to detect toxins in than diet than carnivores or omnivores

  • they encounter more toxins

  • 2TR receptors detect bitter compounds so they are more abundant in plants

• plant secondary metabolites (PSM) include toxins

  • if the animal does not eat the plant with PSM, then there is a cost because it found food but can’t eat it

  • if the animal does eat it, it must be either modified in the gut…

    • metabolized by microbes (detoxify), inactivated in the gut environment (stomach acidity), or modified by endogenous compounds

  • …or if it is not modified in the gut

    • it must be excreted in feces

    • or absorbed which involves oxidation, hydrolysis, methylation, acetylation, glucuronidation, glycine conjugation, and sulfur conjugation

    • but these are very costly

• generalist herbivores have all the absorbance pathways at low levels

• specialist herbivores have one pathway at high levels

  • focus on one particular plant with specific PSMs

Brushtail possum:

• feeds on Eucalyptus

  • melliodora and radiate

• when it is only eating one or the other, it can’t eat more than 25g/kg because its detoxification capacity is low (generalist)

• it can eat more when it eats both because each one produces its own PSM, so the generalist has all of the pathways to digest each type at low levels

  • combined it can have more

Carnivores:

• simple gut → because of the high-quality foods that are easy to digest

• sac-like stomach + tube-like intestine

Herbivores:

• not as simple

• contain fermentation chambers that house large numbers of microbes, which digest refractory compounds

  • ex. cellulose

• the location of the chambers can differ

• Pandas (herbivores) come from carnivore ancestors so it still has the carnivore-like gut

  • they only absorb 10% of what they eat

  • so they are always eating and pooping because they cannot digest but they still need to eat a lot to get enough energy

• a domestic donkey with a fermentation chamber digests 40-70% of what it eats

  • so there is a benefit to having this expensive gut

Cost of Gut Maintenance

• for herbivores to maintain a complex gut is it expensive?

• Burmese python

  • eats once a month

  • small intestine doubles in size after eating a meal → to start digestion

  • and then tears down

  • when measuring the difference in SMR between frequent eaters and infrequent eaters, the SMR is 20-30% lower

  • the maintenance cost of the digestive gut is what drives the 20-30% increase in metabolic rate

• gut epithelial cells:

  • get damaged easily because of foods

  • need to be replaced often

  • when there is lots of thymidine present, more cell division is taking place to rebuild new cells

  • this is what drives the cost of maintaining a gut

Gene eliminations:

• NOX1 is lost in carnivores

• macrophages express NOX1 because they find pathogens and kill them with superoxide

  • superoxide is produced when NADPH Oxidase in NOX1 uses NADPH to synthesize superoxide (damaging to molecules)

• herbivores use NOX1 to prevent bacteria in their fermentation chamber from spilling out and making the body ill

  • they kill the bacteria that get away with superoxide produced from NOX1

  • prevents bacterial overgrowth and spillover

  • In experimental trials, genes with NOX1 knockouts have increased bacterial growth compared to those with NOX1

Week 8 - Lab

Muscles are comprised of cells known as muscle fibers

Red fibers:

• slow oxidative fibers

• Type I fibers

• produce ATP primarily via oxidative phosphorylation

• aerobic pathway

• contract and relax slowly because ATP is